Field of the Invention
The invention relates to a fibre network comprising passive optical networks and sensors.
Background
Passive optical networks are used to transmit optical signals through optical fibres. In passive optical telecommunications networks, optical telecommunication signals are transmitted from a central office of a telecom provider to a number of telecommunications customers or subscribers. A typical passive optical network (“PON”) connects approximately thirty subscribers to the central office, but PONs connecting fewer or many more subscribers exist as well. A typical PON comprises a combined light source/detector device, sometimes called an “Optical Light Terminal” (“OLT”), optical fibres, splitters and optical network units (“ONUs”) at the subscriber locations. The OLT generates telecommunication signals which are transmitted through the optical fibres to the subscribers connected to the PON, and it receives telecommunication signals from the subscribers. At one or two stages of a typical PON, an optical splitter splits the signal of one fibre into several signals which are transmitted in separate optical fibres. Telecommunication signals are thereby cascaded to the ONUs of several subscribers. In order to provide thousands of subscribers with telecommunication services, a great number of PONs are required, each transmitting separate telecommunication signals from the central office to a number of subscribers.
In order to detect external effects like, for example, damage, vandalism, unauthorized access to elements of a PON, or environmental effects, some PONs comprise fibre-optic sensors. These sensors are mostly passive sensors, i.e. they do not require electrical energy to operate. Such fibre-optic sensors are often connected to a central office of the network operator through optical fibres of the PON, either through dedicated sensor fibres, which do not carry an additional communication signal (“dark fibres”), or through fibres that are used for the transmission of communication signals. In response to an external effect, such fibre-optic sensors can modify e.g. the attenuation of the fibre via which they are connected to the central office. Fibre-optic sensors in a PON can be interrogated or “read” by a test signal transceiver, for example by an Optical Time Domain Reflectometer (“OTDR”). For reading a fibre-optic sensor, the OTDR emits an optical query signal into the PON, and measures the intensity of a response signal, which originates from the query signal. Query signals and/or response signals may be called test signals. In order to obtain a stronger response signal, a fibre through which a fibre-optic sensor is connected to other elements of the PON (a “sensor fibre”) is often equipped with a reflector, placed at the end of the sensor fibre and in the vicinity of the sensor. The response signal then originates from the modification, e.g. attenuation, of the query signal by the sensor. In a “normal” state of the fibre-optic sensor, the response signal has, for example, an intensity of 100%, while in an “activated” state of the sensor, the response signal has an intensity of 30%. The time delay between the emission of the query signal and the arrival of the response signal at the OTDR indicates the position of the fibre-optic sensor in the PON and can be used to identify a specific sensor. Interrogating fibre-optic sensors in a PON with such an OTDR technique is advantageous because OTDRs are traditionally used to locate fibre faults in fibre-optic networks and specifically in PONs. This traditional process of locating a fibre fault makes use of the fact that where a fibre is damaged, i.e. at a fibre fault, the signal transmission of the fibre is reduced, and the attenuation is high. The OTDR query signal is thus attenuated at the fibre fault, and the response signal is weaker than a response signal from an undamaged fibre. The OTDR determines the position of the fibre fault from the time delay between emission of the query signal and the receipt of the response signal. A fast OTDR measurement takes about one second, because the OTDR requires a certain amount of time for receiving the response signal and for subsequent data processing. However, due to dynamic range limitations, standard OTDR measurements for fibre fault location take significantly longer, since numerous individual measurements must be performed and averaged. Since fibre faults occur rarely, this time delay is normally not critical. The same OTDR device and the same technique can, in principle, be used to interrogate fibre-optic sensors in PONs.
Some fibre-optic sensors in a PON can be interrogated infrequently, for example a flooding sensor as described in the Japanese patent document JP 2010-212767 A2. Such sensors can therefore be interrogated using traditional, “slow” techniques for monitoring the fibres of a PON. One of those traditional “slow” fibre monitoring techniques is described in the European patent application EP 1980834 A1, where a network monitoring unit determines a failed fibre, and an optical switch is used to select the optical fibre to be tested. An OTDR launches an optical pulse signal into the optical fibre via the optical switch, and receives returning light reflected from reflectors in ONUs of the PON in which the failed fibre is located. It is stated in the document that it takes about 90 seconds per PON to carry out one measurement with a sufficiently high signal-to-noise ratio for easy determination of a point of rupture of the fibre.
In order to protect the network infrastructure effectively, some sensors in a PON must be interrogated more frequently. For example, a sensor for detecting the opening of a door of a splice cabinet of a PON must be read not less frequently than about every 5 to 10 seconds. Otherwise, the door might be opened, an unauthorized activity done in the cabinet, and the door be closed again between two interrogations of the sensor, in which case the opening and closing of the door would remain unnoticed. The traditional technique of optically connecting one test signal transceiver, specifically an OTDR, to one PON via a switch, interrogating the sensors in the PON, then connecting the OTDR to a next PON and interrogating the sensors in this next PON, and so on, until all PONs have been connected to the OTDR and all sensors have been read, cannot provide a frequent-enough interrogation of a specific sensor in a specific PON. On the other hand it would be a very expensive solution to connect a dedicated test signal transceiver, e.g. an OTDR, to each PON, in order to read all sensors frequently enough. It is desirable to interrogate fibre-optic sensors in two or more PONs at a high frequency and using only a single test signal transceiver. The present disclosure seeks to address this problem.